As a poultry keeper I have found that double yolkers occur at the begining of a chickens laying cycle, usually you get very tiny eggs followed by a doudle yolker until the girl gets into her stride. When the commercial growers restock with a few thousand birds at the same time then you will find a proliferation of double yolked eggs many in the same box. 
Chickens that free range and live in normal daylight take 24 -26 hours to produce an egg 
As our poultry expert Cleggy, you dont happen to know much about bantams do you? Just that now weve moved and weve got a bit of garden SWMBO is quite keen on keeping a couple?
The quick answer is no, mine are all pure breed large fowl, Orpingtons, Marans, Welsummers and Wyandottes, however keeping banties is no different except the size. Chickens are easy to keep and reward you with fresh eggs, not the ten day old fresh in supermarkets and their antics provide loads of entertainment. To get a better idea I'll refer you to this site
http://forums.thepoultrykeeper.co.uk/index.phpA bit of reading will give you some good grounding. If you have specific queries please PM me